1. Field of the Invention
This invention relates to a leading mark indicator, more precisely to an instrument for indicating to an observer a definite direction.
2. Description of the Prior Art
A device of the kind here concerned is disclosed previously in U.S. Pat. No. 4,474,430.
Said device comprises a screen in front and a screen behind, which include opaque lines separated by transparent interstices, whereby an interference pattern, so-called moire pattern, arises when an observer views the device. Each of the screens comprises a plate, which is bent centrally to form an obtuse angle, and by attachment means the screen plates are positioned so that the obtuse angles face toward each other. The front screen plate is provided with a screen division which is denser than the screen division of the screen plate behind. Alternatively, the obtuse angles face away from each other, and the front screen plate is provided with a screen division sparser than that of the screen plate behind.
According to said known device, a number of dark interference strips are produced, which are in parallel with each other when the device is viewed in a direction perpendicular to the plane of the screen plates, i.e. the direction or leading mark, which the device is intended to indicate.
When the device is viewed from a direction lying outside the leading mark, which the device is intended to indicate, an interference pattern is formed which consists of a great number of parallel strips, which form a number of arrows, or more properly a fishbone pattern, where interference strips produced by the upper screen surfaces are angular to the interference strips produced by the lower screen surfaces.
Devices of this kind here referred to are known also from U.S. Pat. Nos. 4,166,699 and 3,604,813, which relate to an instrument, at which the moire pattern is utilized for bringing about indications of a certain horizontal or vertical plane for an observer.
The firstmentioned U.S. Pat. No. 4,166,699 relates to a device, at which three screens are utilized, and a predetermined plane is indicated in that a great number of interference strips are in parallel, and the observer being outside said plane is indicated in that an interference pattern including lines with a discontinuous angular deviation arises.
The second one of the aforesaid patents, U.S. Pat. No. 3,604,813, relates to a device, at which two screen plates are arranged angularly relative to each other, and the division of the screens, i.e. the number of opaque lines per length unit perpendicular to the opaque lines, is chosen so that an interference pattern arises, which is intended to be read in respect of the pattern symmetry. It is extremely difficult to accurately determine the indicated plane by means of such a pattern.
All of said known devices, thus, are designed to form a great number of interference strips, the angles of which in one direction or the other form discontinuous angle variations or asymmetric patterns when an observer is outside the plane or direction, which the device is intended to indicate.
The total width of opaque lines normally is such that it exceeds or is substantially equal to the total width of transparent interstices.
This implies that such an instrument is difficult to read, especially on a large distance. The reasons substantially are two. A first reason is that an observer is met only by the light which is transmitted through the screens, i.e. light of a surface of about half the device surface perpendicular to the viewing direction.
It is, for example for navigation purposes, essential that as great a proportion as possible of luminous surface is obtained in order to increase the visibility of the device, especially at a long distance or in bad weather.
A second reason is due to the fact that a large number of dark interference strips and light interspaces by necessity gives that each interference strip and each interspace is relatively narrow in comparison with its length, wherereading of the interference pattern is made more difficult, especially at greater distances or in bad weather.